A power supply is generally regarded as an electrical device which converts electrical power of one characteristic to electrical power of another characteristic. Power supplies are typically used to convert commercially available alternating current (AC) power to direct current (DC) power, and to convert DC power to AC power. Sometimes, a power supply will perform two or more conversions to obtain the desired level or characteristic of electrical power. Conversions such as these may be desirable or necessary for a wide variety of reasons, most of which are well known and appreciated and many of which are made desirable because of the characteristics of the particular device which is to be powered. Furthermore, many of the operational concepts of power supplies are well known, and it is improvements to these operational concepts to which the present invention pertains.
One type of converter is a resonant converter. A resonant converter employs a resonant circuit formed by capacitive and inductive elements in the primary winding circuit of a power transformer. Current in the resonant primary winding circuit alternates at a natural frequency established by the values of its capacitance and inductance. The primary winding current induces a flux in the transformer which alternates at the same frequency as the natural resonant frequency of the primary winding current. The flux causes the secondary winding circuit of the transformer to produce voltage and current which alternates at the same frequency as the primary winding current. The output voltage and current from the transformer are established by the ratio of the number of primary and secondary windings of the transformer. The energy supplied to the resonant primary circuit is ultimately converted to electrical output power, with some power consumed by losses. The energy conversion and parasitic losses cause the primary resonant current to decay, thus requiring the resonant primary circuit to be re-energized.
Resonant converters are of a continuous or discontinuous type, depending upon the manner in which the resonant primary circuit is re-energized. A continuous resonant converter continually supplies re-energizing current to the resonant primary circuit. As a consequence, the current in the resonant primary circuit is varied only enough to obtain the degree of regulation desired, without decaying or fluctuating substantially.
A discontinuous resonant converter intermittently switches DC current to the resonant primary circuit to re-energize it. In general, discontinuous resonant converters do not re-energize the resonant primary circuit in complete coincidence with or in phase with the electrical current resonating in the primary winding, because the naturally alternating current decays substantially or completely before the primary circuit is re-energized. The power delivery characteristics of the discontinuous converter are regulated by controlling the time duration between the intermittent intervals of power delivery to the resonant primary circuit. When more output power is desired, the time between intermittent energy delivery to the resonant primary circuit is decreased, and vice versa.
Discontinuous resonant converters are not preferred in situations where a high degree of precision in regulating the output power level is required or desired. The intermittent nature of the operation of the discontinuous resonant converter results in relatively significant changes in the instantaneous power delivered over time, thus making the highest degree of regulation impossible or difficult. In those cases where a discontinuous resonant converter is used and a relatively high degree of power delivery is required on a continual basis, the output AC power is generally converted to DC power by a rectifier and storage capacitors. The storage capacitors are usually of a considerable size to absorb and compensate for the significant fluctuations in power delivery from the discontinuous resonant converter. Furthermore, the size of the transformer and the current switches supplying current to the resonant primary circuit are of a greater capacity and thus more costly because they must deliver relatively greater quantities of power over relatively shorter time periods.
Since continuous resonant converters offer the capability of achieving a more uniform power delivery on a continuous basis, the size and cost of the components can be reduced. However, most of these considerations have been recognized only in theory because the practical difficulties of implementing continuous resonant converters have been substantial. One of the significant difficulties has been the regulation of power. It has been very difficult to add energy to or remove energy from the resonant primary circuit on a sufficiently responsive and precise basis to obtain a relative high degree of control over the output power.
One theoretical approach to reducing power in a continuous resonant converter is to switch current in opposition to the current naturally oscillating in the primary winding. The opposing current reduces the magnitude of the primary circuit current and thus reduces the power output. Another approach is to switch current at a frequency which is slightly different than the natural frequency. Because the switched current is slightly out of phase with the naturally alternating current, the resonant effects of the resonant primary circuit decay, resulting in a corresponding reduction in output power. The practical difficulty with both approaches of power reduction is that the phase difference between the switched current and the naturally oscillating current requires the current switches to absorb significant amounts of power during switching. The power absorbing requirement is so substantial that there is a risk of destroying the switches.
Since a well regulated power supply is continually increasing and decreasing power output to achieve adequate power regulation, the problems in power absorption have been significant impediments to the successful implementation of continuous resonant converters. Because of these limitations, pulse width modulated (PWM) driver circuits are frequently usually used to drive the primary winding of the power transformer, rather than a resonant circuit. In PWM power supplies, the amount of current supplied by the switches of the PWM driver is more easily controlled.
Another difficulty with continuous resonant converters is the capability of quickly regulating the quantity of current switched to the primary resonant current. Since the efficiency of power transfer is increased at higher operating frequencies, due to the better coupling of the flux between the primary and secondary windings in practical transformers at the higher frequencies, the operating frequency of high efficiency power supplies is generally very high, for example up to 100 kHz. While high-power, relatively-fast-operating, transistor current switches, such as metal oxide semiconductor field effect transistors (MOS-FETs), are capable of switching sufficient current at the relatively high frequencies, it is considerably more difficult to obtain sufficiently responsive control signals by which to control the current switches. The control signals are usually feedback signals which are derived by comparing a signal representative of an output characteristic, such a voltage or current, with a predetermined signal representative of a desired amount of the output characteristic. There is an inherent lag time between the response of the power supply and the determination that an output condition should be corrected. A reduced lag time response is desirable because it results in a higher degree of precision in regulation, which is desired or required in many applications.
One particularly demanding application for a power supply is controlling an x-ray tube in an x-ray device. The x-ray power supply must have the capability to adjust the level of output voltage to assure the best quality image for the particular type of body part being imaged. Higher voltages are required for x-raying more dense body parts such a bone, and lower voltages may be used adequately for x-raying soft tissues such as organs.
The quality of an x-ray image is directly related to the precision of regulation and the level of voltage applied to the x-ray tube. Higher applied voltages cause the x-ray tube to emit greater quantities of "hard" or high energy radiation. The hard radiation causes a good image by increasing the contrast with less exposure time. Minimum exposure times are desirable to reduce the potentially damaging effect of x-rays on the human body. Lower applied voltages generate levels of "soft" radiation which is of little assistance in creating a good image but adds to the total exposure time. Establishing and maintaining the desired level of output voltage obtains the best quality of image and the minimum amount of exposure time for each particular application.
An x-ray power supply must rapidly change or adjust the output voltage in short times. Most x-ray tubes are triggered by rapidly increasing the voltage across the x-ray tube from a level where no radiation is emitted, to a level where the x-ray tube is triggered into conduction. Conductivity of the x-ray tube is terminated by rapidly decreasing the voltage. The increase in voltage to trigger the x-ray tube and the decrease in voltage to terminate the emission of radiation should occur as rapid as possible, to minimize the emission of soft radiation and to control the exposure time more precisely. The high voltages applied to the x-ray tube, for example 150,000 volts, may cause undesired arcing across the x-ray tube. Arcing conditions can dissipate an explosive amount of energy and degrade the x-ray tube. Under arcing conditions it is desirable to almost instantaneously terminate the supply of output voltage.
The x-ray power supply should also have a capability to respond rapidly and effectively to maintain the output voltage level as close as possible to the desired level. Maintaining the voltage at the desired level avoids ripple and fluctuations in voltage which could vary the quality of the image and possibly result in the emission undesirable quantities of soft radiation. Some x-ray imaging applications require a narrow spectrum of emitted radiation, and low ripple or variation across the x-ray tube is very important in producing the narrow spectrum of radiation. Triggering the x-ray tube into conduction changes the load connected to the power supply, which further increases the difficulty of maintaining the desired output voltage. Good responsiveness in output voltage regulation is very important in the satisfactory performance of x-ray power supplies.
To increase the voltage output level, reduce the ripple and to obtain more responsive regulation, higher frequency converters are desirable. For example the best currently available x-ray power supplies operate at a conversion frequencies in the range of about 80 to 100 kHz.
It is with respect to this general background information, and other more specific information not specifically discussed herein that the present invention has resulted.